A forward-adjoint operator pair based on the elastic wave equation for use in transcranial photoacoustic tomography

TL;DR

This paper develops a GPU-accelerated elastic wave equation-based forward and adjoint operator pair for transcranial photoacoustic tomography, enabling accurate modeling of skull effects in brain imaging.

Contribution

It introduces a finite-difference time-domain elastic wave model and its adjoint, optimized for GPU computation, to improve transcranial PACT image reconstruction.

Findings

Validated with computer-simulation studies

Demonstrated effectiveness in phantom experiments

Enhanced modeling of skull-induced wave effects

Abstract

Photoacoustic computed tomography (PACT) is an emerging imaging modality that exploits optical contrast and ultrasonic detection principles to form images of the photoacoustically induced initial pressure distribution within tissue. The PACT reconstruction problem corresponds to an inverse source problem in which the initial pressure distribution is recovered from measurements of the radiated wavefield. A major challenge in transcranial PACT brain imaging is compensation for aberrations in the measured data due to the presence of the skull. Ultrasonic waves undergo absorption, scattering and longitudinal-to-shear wave mode conversion as they propagate through the skull. To properly account for these effects, a wave-equation-based inversion method should be employed that can model the heterogeneous elastic properties of the skull. In this work, a forward model based on a finite-difference time-domain discretization of the three-dimensional elastic wave equation is established and a procedure for computing the corresponding adjoint of the forward operator is presented. Massively parallel implementations of these operators employing multiple graphics processing units (GPUs) are also developed. The developed numerical framework is validated and investigated in computer-simulation and experimental phantom studies whose designs are motivated by transcranial PACT applications.